For some video applications, a video stream needs to be transmitted to multiple endpoint devices simultaneously. Such a video stream is referred to as a “multicast” stream and the devices that are to receive the stream are part of a multicast group. Sending multicast video in a wireless network, such as an IEEE 802.11 wireless local area network (WLAN), presents certain challenges.
The bandwidth allocated for multicast transmissions on an IEEE 802.11 radio link is a fraction (i.e., 2%) of the bandwidth allocated for multicast transmissions on a wired Gigabit Ethernet link, for example. Multicast frames are sent at a “basic rate” on IEEE 802.11 radio links. For example, the highest basic rate for an IEEE 802.11a/b/g/n access point (AP) is typically 24 Mbps. The available bandwidth of an 802.11 link can be highly variable, primarily due to overlap contention.
In an enterprise network, a single multicast video stream may be destined to diverse computer platforms that are connected both via high-bandwidth wired Ethernet links and relatively low-bandwidth wireless IEEE 802.11 links. For example, the same multicast Internet Protocol TV (IPTV) stream may be received by a wired Ethernet High Definition TV (HDTV) monitor and a wireless IEEE 802.11 hand-held device.
The frame error rate on 802.11 wireless links is orders of magnitude higher than the frame error rate on wired Ethernet links. Multicast frames are unacknowledged on 802.11 links. Therefore, a relatively high percentage of multicast frames are lost on 802.11 wireless links. Most video encoding standards are susceptible to frame loss, and they do not facilitate software-based transcoding.